20 



the battery is of very common occurrence in mineral veins ; and 

 even the same vein presents large and small crystals, and often 

 of variable composition, within a very small compass. 



The application of the agency of fire to form such depositions, 

 is not only a rude and clumsy hypothesis, but totally inconsist- 

 ent with analogy, and contrary to facts : whereas a magnetic or 

 galvanic current passing through solutions, if not the actual 

 modus operandi of nature, is at all events capable of giving a ra- 

 tional solution, not merely of the mineral veins, but also of all 

 the operations of nature disclosed by geology. 



In order to understand how the metals are reduced from their 

 solutions by the agency of a galvanic current, let us take a bat- 

 tery, and endeavour to discover the cause of the actions there 

 going on. 



In placing a plate of zinc in water, and allowing it to be im- 

 mersed in it for a long period, it becomes coated with an oxide ; 

 but if the water be diluted with sulphuric acid, the oxide will be 

 dissolved, and the zinc will continue to present a clean surface 

 to the oxygen of the water, until it is entirely dissolved, provided 

 there be a sufficient supply of that element present. 



The above chemical action is however comparatively feeble. 

 This feebleness of action appears to arise from the want of an- 

 other power to take away the hydrogen evolved during the de- 

 composition of the water by the zinc. If we place another metal 

 having a less affinity for oxygen than the zinc in the same vessel, 

 and connect the two by a copper wire, the action is considerably 

 increased, and the hydrogen evolved from the decomposition 

 of the water is apparently conveyed from the zinc by means of 

 the connecting wire, and finally escapes from the surface of the 

 plate which forms the negative. And it has been experimentally 

 proved that the greater the facilities by which the hydrogen is 

 made to evolve from the negative plate, the greater is the action 

 on the zinc or positive plate. 



The positive plate may be considered as the fuel of the bat- 

 tery, and the connecting wire, with its negative plate, the flue 

 and chimney of the battery. If we increase the draft of the 

 chimney for the escape of the decomposed air, the greater is the 

 supply of oxygen to the fuel ; if we break the connexion be- 



